Tom Lane wrote:

Kevin Brown <[EMAIL PROTECTED]> writes:


Hmm...something just occurred to me about this.


Would a hybrid approach be possible? That is, use mmap() to handle
reads, and use write() to handle writes?



Nope. Have you read the specs regarding mmap-vs-stdio synchronization? Basically it says that there are no guarantees whatsoever if you try this. The SUS text is a bit weaselly ("the application must ensure correct synchronization") but the HPUX mmap man page, among others, lays it on the line:

    It is also unspecified whether write references to a memory region
    mapped with MAP_SHARED are visible to processes reading the file and
    whether writes to a file are visible to processes that have mapped the
    modified portion of that file, except for the effect of msync().

It might work on particular OSes but I think depending on such behavior
would be folly...

We have some anecdotal experience along these lines: There was a set of kernel bugs in Solaris 2.6 or 7 related to this as well. We had several kernel panics and it took a bit to chase down, but the basic feedback was "oops. we're screwed". I've forgotten most of the details right now; the basic problem was a file was being read+written via mmap and read()/write() at (essentially) the same time from the same pid. It would panic the system quite reliably. I believe the bugs related to this have been resolved in Solaris, but it was unpleasant to chase that problem down...

-- Alan

---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
     joining column's datatypes do not match

Reply via email to